1. Field of the Invention
The present invention relates to a pressure sensor, and in particular, to a pressure sensor that detects pressure difference between pressure of fluid to be measured and the atmospheric pressure.
2. Description of Related Art
Conventionally, in order to measure a fluid pressure, there has been used a pressure sensor that has a pressure receiving surface for fluid to be measured and an atmospheric pressure receiving surface, and is provided with a pressure sensor element detecting pressure difference between pressure of fluid to be measured and the atmospheric pressure to convert the pressure difference into electrical signals and a main body case that contains the pressure sensor element therein.
There is known a pressure sensor element with structure using a diaphragm. That is, one side surface of the diaphragm constitutes the pressure receiving surface for fluid to be measured and the other side surface constitutes the atmospheric pressure receiving surface. The fluid to be measured is brought into contact with the pressure receiving surface for fluid to be measured, and the atmospheric pressure receiving surface is opened to an internal space of the main body case where the pressure is kept to be same as an atmospheric pressure. Having the construction as described above, the diaphragm is deformed by the pressure difference between the pressure of fluid to be measured and the atmospheric pressure. The deformation of the diaphragm is detected as change of capacitance, electrical resistance or the like generated by the deformation and the pressure difference is converted into electrical signals.
In such a pressure sensor as described above, it has been conventionally general that the main body case employs sealed structure to improve a waterproof and a dust-proof function so as to protect the pressure sensor element. Sealing the main body case, however, brings about pressure change of the internal space of the main body case due to temperature change or diaphragm deformation, then pressure applied to the atmospheric pressure receiving surface of the diaphragm becomes unequal to the atmospheric pressure resulting in a problem of measurement accuracy deterioration.
In order to solve the above problem, there is a method that the main body case is not arranged to have sealed structure, but a vent is provided to communicate the internal space of the main body case with the atmosphere. In this method, however, when a measurement environment is severe where the pressure sensor is used, water or dust or the like enters into the internal space of the main body case through the vent and directly adheres to the pressure sensor element. As a result, such a problem is generated that the operation of the pressure sensor element fails, measurement accuracy deteriorates or measurement becomes impractical.
In order to solve above problems, there is known a method to overlay the vent communicating the internal space of the main body case with the atmosphere by means of a filter having air permeability and a waterproof property (for instance, refer to Japanese Patent Laid-Open Publication No. 11-148880, FIG. 1 and others). Since the filter has the air permeability, the pressure of the internal space of the main body case is kept equal to the atmospheric pressure. Moreover, since the filter has the waterproof property, it is possible to prevent water from entering into the internal space of the main body case.
In this method, however, it is necessary to provide a filter to prevent water ingression into the internal space of the main body case, which generates problems of cost increase due to a filter material cost and a filter installation cost, as well as complication of the structure.